This invention relates to an image forming apparatus and, more particularly, to an image forming apparatus designed such that the document table is allowed to make its reciprocating movement in accordance with the running of a chain.
In a conventional copying machine, the document table is required to be moved at a low and unvaried speed at the time of exposure. For example, after a document is subjected to a light exposure in a forward stroke of the document table, the document table is moved at high speed in its backward stroke in order to shorten a copy cycle. Thus the copying machine is usually equipped with at least two spring clutches, one for a low speed mode and the other for a high speed mode, and the clutch operation is performed to transmit a driving force to the chain through a selected one of the clutches.
However, the spring clutch has a response time from receipt of a start instruction to reliable transmission of a driving power that is longer than the response time for receipt of a stop instruction. For this reason, when, at the time of switching the movement of the document table from its forward stroke to its backward stroke, the stop and start instructions are simultaneously applied to the low and high speed clutches respectively, the chain runs temporarily without sufficient power from either spring clutch. For this reason, for example, pressing the document, e.g., a thick book, on the document table by hand would cause a displacement of the document table due to the pressing force, causing a miscopy.
Most of the prior art copying machines, therefore, adopt the following system for the purpose of preventing such an idle displacement of the document table. That is, whenever the document table is moved, the spring clutch for low speed is actuated, and, in a state wherein a driving force is supplied to the chain, a start instruction or stop instruction for change-over of the speed is applied to the spring clutch for high speed only. In this system, since driving force is supplied by the spring clutch for low speed to the chain during a period of time in which a sufficient power is not transmitted to the chain by the spring clutch for high speed, no idle movement occurs in the document table. Thus, when the spring clutch for high speed is actuated, the chain falls under the control of this spring clutch and is thus allowed to travel at high speed.
In such a system, however, since a large load is applied to the spring clutch for low speed, the service life of the same is shortened. Further, while control of a copying operation required knowledge of the position and the direction of movement of the document table, it was impossible with the prior art copying machine to know the direction of table movement by the use of the magnets mounted on the document table.